import React ,{useState} from 'react';
import {useEffect} from 'react'
export default function App(){
    let [opacity,setOpacity]=  useState(1)
    useEffect(()=>{
        const opacityTimer = setInterval(()=>{
            opacity-=0.1
            if(opacity <=0){
                opacity=1
            }
            setOpacity(opacity)  
        },100)
        return () => {
            clearInterval(opacityTimer)
          }
    },[])
    return (
        <div>
          <h1 style={{ opacity }}>ReactHook 真好用</h1>
        </div>
      )
}



